The present invention relates to a method of operating a device such as a memory, a testing device for a memory and to a method of determining a minimum sense amplifier clock delay.
Semiconductor memories have a matrix of memory cells, the matrix having rows formed by wordlines and columns formed by complementary pairs of bitlines. The bitline pairs are connected to clocked sense amplifiers which evaluate the contents of a cell which is connected to the bitlines. The complementary bitlines tend to have a high capacitance and the memory cells tend to have a relatively low current driving capability which means that the differential voltage across the complementary pair of bitlines increases relatively slowly.
An ideal sense amplifier would be capable of connection to the bitlines shortly after the memory cell was connected to them and would then respond to the increasing differential voltage to provide either a logic 1 or logic 0 output. However, sense amplifiers normally have an input offset. This offset, which may be positive or negative, must be overcome by the differential voltage on the bitlines before the sense amplifier can latch into the correct state.
For security it is therefore common practice to delay clocking the sense amplifier until a large differential voltage has developed across the complementary bitlines. This allows for manufacturing tolerances. It also avoids the need for measuring the minimum time needed for correct sensing to occur. Such a measurement is difficult especially where the memory is embedded.
The present invention aims to provide a technique for determining the minimum sense amplifier clock delay, which minimum delay still gives rise to a correct output from the sense amplifier.
According to a first aspect of the present invention there is provided a method of operating a device, the device comprising circuitry having an input responsive to a first timing signal and producing a valid output to clocked output circuitry after a first delay, said clocked output circuitry having a clock node, and a clock delay circuit responsive to said first timing signal for providing at a second delay, a second timing signal to said clock node of said clocked output circuitry, the method comprising repeatedly applying said first timing signal followed by an external timing signal, wherein said external timing signal is applied via a delay-producing first path to said clock node; monitoring an output of said clocked output circuitry; varying the delay of application of said external timing signal with respect to said first timing signal; and in response to a desired output of said clocked output circuitry, modifying said second delay.
Preferably said desired output is a just valid output such that any reduced delay in said external timing signal gives rise to an invalid output.
Advantageously before said repeatedly applying step said method further comprises determining when said second timing signal occurs and, during said repeatedly applying step, determining when said desired output occurs.
Conveniently said step of determining when said second timing signal occurs comprises providing latch circuitry responsive to the output of said clocked output circuitry, said latch circuitry having a latch clock node connected to a latch clock pad via a delay producing second path; successively providing said second timing signal to said clock node of said clocked output circuitry and a latch timing signal to said latch clock pad; varying the time of occurrence of said latch timing signal; and monitoring the latch circuitry output.
Advantageously after said step of monitoring the latch circuitry output, the method further comprises applying the externally generated clock signal via the delay-producing first path to said clock node and varying the time thereof; and monitoring the latch circuitry output.
Conveniently said device is a memory comprising an address register having a clock input, an array of memory cells having wordlines connected to said address register, and plural bitlines and clocked sense amplifiers for evaluating said bitlines.
According to a second aspect of the present invention there is provided a method of determining a minimum sense amplifier clock delay in a memory, the memory comprising address register circuitry having a clock input, an array of memory cells having wordlines coupled to said address register circuitry and bitlines connected to sense amplifier circuitry, said sense amplifier circuitry having a clock input, the memory further having output circuitry having an input coupled to the output of said sense amplifier circuitry and an output, the method comprising:
providing a first path having an input, said first path having an output coupled to said clock input of said address register circuitry;
providing a second path having an input, said second path having an output coupled to said clock input of said sense amplifier circuitry;
repeatedly applying first timing signals at said input of said first path and at a variable delay after each said application, applying a second timing signal at said input of said second path; and
evaluating conditions at said output of said output circuitry.
Preferably said output circuitry has an output circuitry clock input, said memory further comprises a sense amplifier clock generator having an output, and said method comprises, before said repeatedly applying step, the repeated steps of applying first timing signals at said first path, and applying the output of said sense amplifier clock pulse generator to said clock input of said sense amplifier circuitry whilst varying the delay of application of a timing signal with respect to said first timing signal to said clock input of said output circuitry until a desired output condition exists.
Advantageously said repeatedly applying step comprises applying said timing signal to said output circuitry clock input at the delay where said desired output condition exists.
Advantageously said desired condition is a condition where an output is valid, but any reduction in said delay gives rise to an invalid output.
According to a third aspect of the present invention there is provided a testing device for a memory, said memory having address register circuitry having an address register clock node, a memory array and sense amplifier circuitry having a sense amplifier clock node and a sense amplifier output, said testing device comprising a first timing signal generator coupled to said address register clock node via a first path for clocking said address register circuitry, a sense amplifier clock generator responsive to a signal at said address register clock node for producing a first sense amplifier clock signal, a second timing signal generator coupled to said sense amplifier clock node for providing a second sense amplifier clock signal, clocking and selector circuitry for selectably coupling said first or said second sense amplifier clock signal, to said sense amplifier clock node, said device further comprising means for varying the relative timing of said first timing signal with respect to said second timing signal.
Advantageously said testing device further comprises output circuitry connected between said sense amplifier output and a device output, and evaluating apparatus coupled to said device output.
Conveniently said output circuitry comprises clocked latch circuitry connected at said sense amplifier output, and having a clock node coupled to receive a third timing signal.